A continuing program in laboratory animal nutrition involves study with the most frequently used species of small laboratory animals such as rats, mice, guinea pigs, and rabbits as well as dogs and various species of nonhuman primates. A series of factorial designed feeding trials are used to ascertain the nutrient requirements of the animals of interest and to develop diets with more nearly optimal nutrient concentrations for their growth, reproduction, maintenance, or general health status. This program has resulted in the development of open formula natural ingredient diets for most of the animal species maintained in significant numbers at NIH. The major exception is the dog and plans have been made to initiate studies involving this species. In addition, a major effort is being made to develop a complete diet for marmosets.